A shifting device for connecting components of a transmission that are rotating at different speeds, is known from DE 197 56 637 A1. For each shifting group of a range-change transmission, a unit is provided in the shifting device, in which valves, a shifting cylinder, a piston and shift elements are combined. Pulsed 2/2-way valves are used as release valves. In addition, in each case a displacement measuring device is provided, by means of which the current position of a piston in relation to a shifting cylinder can be indicated. Thus, when gearwheels are engaged in torque-transmitting connection, the possibility exists of adapting the shifting force and the shifting time.
To shift a shifting piston in the shifting cylinder, a pneumatic or hydraulic pressure medium from an external pressure source is delivered, via pressure lines, to the shifting device. In each of the pressure lines are interposed two electromagnetic 2/2-way valves, which control the flow of the pressure medium through the pressure lines in such manner that the shifting piston moves at a speed which corresponds to an optimum speed in the displacement position concerned, determined empirically or by a simulation.
For this, the control of the pulsed valves is designed such that all of the valves can be activated individually, in opposition to, or together with one another. Thereby various shifting piston speeds and thus also differing dynamic forces can be obtained.
The shifting piston speed is varied by two respective 2/2-way pulsed valves in such manner that during its shifting displacement, up to the beginning of synchronization in the transmission the shifting piston is moved as much as possible under full pressure force, and after the end of the synchronization phase, it is brought to its end position at a lower speed.
On entering a synchronization phase of the transmission, shortly before encountering a stop surface the shifting piston pauses in a retention position such that in this position the pressure acting on the shifting piston increases considerably due to a further pressure medium pulse via the pressure medium lines.
With the shifting device proposed, a variation of the synchronization times is possible by virtue of the electronic control of the pulsed valves. Thus, the synchronization processes for shifts on level ground, downhill gradients and uphill gradients can be carried out differently. During shifts on level ground the synchronization devices can be operated as gently as possible. On downhill gradients the synchronization devices can be subjected to a higher pulse frequency and used up to a maximum load, which can there lead to safety-relevant, shorter shifting times. The shifting speeds can be optimized thanks to the variability of the shifting and synchronization forces.
To avoid shifts when there is a tooth-on-tooth position between the respective tooth profiles to be connected, it is proposed to monitor such an operating situation. If a tooth-on-tooth position occurs, the synchronization force acting on the shifting piston during the synchronization phase is minimized, in order to produce a drag torque in the synchronization associated with the positive-locking shifting element and thereby release the tooth-on-tooth position in the area of the positive-locking shifting element.
A disadvantage of this, however, is that the mechanical synchronizers, provided for synchronizing positive-locking shifting elements, take up considerable structural space and are characterized by substantial complexity of design as well as control and regulation.
In contrast to shifts in which at least one positive-locking shifting element that can be synchronized by a mechanical synchronizer is involved, shifts or powershifts carried out at least with one purely positive-locking shifting element without any associated mechanical synchronizer cannot be carried out to the desired extent with reproducible shifting quality and within predefined shifting times since, as is known, the closing point of purely positive-locking shifting elements cannot be reached to the desired extent at a predefined time point by conventional methods.